1. Field of the Invention
The present invention relates to a radiographic apparatus for photographing a radiographic image of an object in the medical field or nondestructive inspection field.
2. Related Background Art
(1) FIG. 1 is a diagram of a conventional radiographic apparatus. A radiographic image photographing means 302 for photographing a transmitted radiographic image of an object S is disposed in front of a radiation generating means 301 as a radiation source for generating radiation. The radiation generating means 301 and radiographic image photographing apparatus 302 irradiate radiation and photograph an image of the object S on the basis of the photographing conditions, e.g., the tube voltage or tube current for an X-ray tube as the radiation source, irradiation time, and the like, set by the operator using a photographing condition setting means 303.
(2) In X-ray radiography, X-rays that have scattered inside an object largely influence the image. In order to efficiently remove scattered X-rays, a scattered X-ray removing grid (to be simply referred to as a grid hereinafter) is used to help improve the contrast and sharpness of an X-ray image. The grid used can be classified into a parallel grid and convergence grid depending on their structures. FIG. 2 is a sectional view of the parallel grid, in which copper foils 391 and intermediate substances 390 are disposed to be parallel to each other and in a direction perpendicular to incoming X-rays. FIG. 3 is a sectional view of the convergence grid, in which copper foils 391 and intermediate substances 390 are disposed to converge at a single point (in this case, a convergence point 401). The intermediate substance consists of aluminum, wood, or the like.
(3) Conventional radiography uses a system as a combination of a film and intensifying paper. In recent years, along with the development of computers, various types of digital image photographing apparatuses have been developed and are used in clinical applications. A photographing apparatus using a photostimulable phosphor sheet as one of such apparatuses temporarily records a radiographic image of an object S on a photostimulable phosphor sheet, and then irradiates excitation light such as a laser beam onto that photostimulable phosphor sheet to cause stimulated emission. Based on an image signal obtained by photoelectrically reading the emitted light, a radiographic image of the object S is printed on a silver halide film or is displayed on a CRT display.
On the other hand, a photographing apparatus using a photodetection array converts a radiographic image of the object S into a visible image via a scintillator or image intensifier, converts that visible image into an image signal via the photodetection array, and prints or displays the radiographic image of the object on a silver halide film or CRT display.
(4) Furthermore, in radiography in the medical field, in order to obtain a high-quality image without re-photographing, the radiographic conditions must be set to match the state and characteristics of the object S. That is, the field of irradiation, quality, and exposure dose of radiation must be optimized, and appropriate image processes are required for a digital radiographic image to make it easier to see.
FIG. 4 shows the arrangement of a radiographic apparatus according to the third conventional art. When a radiation generating means 301 irradiates radiation onto an object S, the radiation is intensity-modulated and scattered in accordance with the internal structure of the object S owing to interactions such as absorption, scattering, and the like of the object S with respect to the radiation, and then reaches a radiographic image photographing means 302 to obtain a radiographic image. Note that a grid 304 disposed in front of the radiographic image photographing means 302 removes scattered radiation to improve the contrast of the radiographic image.
In general, the radiographic image photographing means 302 comprises a phosphor CaWO4 or the like that produces luminescence at an intensity proportional to the exposure dose, and a silver halide film, and the image of the object S is recorded on the film as a latent image. After development, the recorded image is presented as a visible image that gives a density proportional to the logarithm of the luminescence amount, and is used in diagnosis, inspection, and the like.
Also, a computed radiography (CR) apparatus using an imaging plate applied with a BaFBr:Eu phosphor and BaF:Eu phosphor which produce photostimulated luminescence is also used. The CR apparatus temporarily records a radiographic image of the object S on the imaging plate, and then irradiates excitation light such as a laser beam onto the imaging plate to cause stimulated emission. The apparatus prints or displays the radiographic image of the object S on a silver halide film or CRT display on the basis of an image signal obtained by photoelectrically reading the emitted light.
Furthermore, recently, a technique for reading a digital image using, as the radiographic image photographing means 302, a photoelectric conversion device on which pixels each consisting of a very small photoelectric conversion element, switching element, and the like are arranged in a lattice pattern, has been developed.
(5) It is important in radiography to obtain a high-quality image without re-photographing, and optimal radiographic conditions must be selected in correspondence with the state and characteristics of the object S and those of the radiographic apparatus. That is, the field of irradiation must be stopped down, and the dose and quality of radiation must be optimized. Furthermore, when a radiographic image is to be digitally processed, posture determination, edge extraction, and the like of the object S are required.
In order to stop down the field of irradiation, a lead aperture stop is conventionally inserted immediately after the radiation generation device, and is manually moved. In order to confirm the divergence of radiation, a visible light source is arranged at a position conjugate with the radiation generating means 301, and the operator visually checks the degree of eclipse of the projected light by the aperture stop. In addition, in an X-ray radiography apparatus, the irradiation range is confirmed in advance using a television monitor.
Upon setting the dose and quality of radiation, the photographer sets them by determining proper conditions on the basis of the posture and photographing portion of the object S, or inputs information associated with the posture and the photographing portion of the object S to the apparatus, which automatically sets proper conditions.
(a) However, in conventional art (1) above, since the operator must set optimal photographing conditions to obtain a radiographic image which is easy to observe, he or she must change the positional relationship between the radiation generating means 301 and radiographic image photographing means 302 depending on the photographing method used, and must measure the distance between them using a scale in every change. Furthermore, before the operator gains experience in using the apparatus, e.g., immediately after installation of the photographing apparatus, he or she must create an irradiation condition table or the like and must photograph with reference to that table. Upon creating the irradiation condition table, the operator must make physical contact with a patient as the object S to directly measure the breast thickness using a tool such as a breast meter or the like.
(b) When the grid described in conventional art (2) is used, grid cutoff occurs. FIG. 5 shows the case wherein grid cutoff has occurred due to the parallel grid, and illustrates an X-ray tube focal point F, and shadow images 414a and 414b on an image receiving surface 413 obtained when X-rays are transmitted through lead foils 412a and 412b of a grid 411. The lead foils 412a is projected as a shadow image broader than that of the lead foil disposed in the direction of primary X-rays, which do not reach the image receiving surface accordingly. As a consequence, in an X-ray image, a portion where the broader shadow image is formed becomes darker than a portion where it is not formed. The grid cutoff amount normally becomes larger as the grid ratio is higher and the distance between the grid and X-ray tube focal point F is shorter.
Even when the convergence grid is used, if the positional relationship between the X-ray tube focal point F and a convergence point 401 of the grid is not proper, grid cutoff takes place. FIG. 6 shows an example wherein the X-ray tube focal point F deviates horizontally from the convergence point 401. In this case, since all the lead foils of the grid cause equal grid cutoff of primary X-rays, an entirely and evenly dark X-ray image is obtained.
The grid is classified into a still grid and moving grid depending on their use methods. The still grid is used in the still state with respect to an X-ray image receiving surface (to be simply referred to as an image receiving surface hereinafter), and the moving grid is built and used in a device for moving the grid relative to the image receiving surface to remove shadow images of the lead foils.
Even in a conventional apparatus using the moving grid, since the moving speed of the grid is constant, the grid does not move at equal intervals about the center of the field of X-ray irradiation during the X-ray irradiation time, and the same result as that obtained by integrating the horizontal deviation state within the irradiation time is obtained, i.e., a density pattern is formed in an X-ray image. On the other hand, when the photographing time is long and the grid must be reciprocally moved, shadow images of the lead foils are often formed at the turning point positions.
(c) In conventional art (3) above, an apparatus which comprises image process means for outputting an image with an optimal density and contrast when overexposure or underexposure has occurred due to condition setting errors upon photographing, or an apparatus which comprises determination means for determining the photographing posture, photographing portion, and field of irradiation of the object S to optimally execute such image process is known. However, since the radiographic image of the object S is used in such determination, the image size is as large as 1024xc3x971024 samples and 12 bits required for quantization, thus requiring a long arithmetic time. Also, under the influences of scattered radiation, it is hard to accurately execute pattern matching of the object S and recognition of the field of irradiation.
(d) In conventional art (4) above, in order to stop down the field of irradiation of radiation coming from the radiation generating means 301, a movable radiation aperture stop 305 inserted immediately before the radiation generating means 301 is manually adjusted. Furthermore, a light source 306 is arranged at a position conjugate with the radiation generating means 301, and the operator confirms the field of irradiation by visually observing the degree of eclipse of the projected light by the movable radiation aperture stop 305. In this case, the operator must stand at the side of the radiation generating means 301, and must adjust the width of the movable radiation aperture stop 305 every time the object S changes, thus requiring very troublesome operations. Especially, in case of breast photographing, since front and side images of an identical object S must be alternately photographed, the operator must adjust the width of the aperture stop in each photographing. Owing to such tedious operations, the operator may often photograph a side image with a small width without stopping down the movable radiation aperture stop 305.
However, when the side image of the object S is photographed without stopping down the movable radiation aperture stop 305, i.e., in the full-open state, radiation also reaches an ineffective photographing region, and so-called unintercepted radiation, which is not absorbed by the human body, directly reaches a photo-timer light-receiving unit 307 used for automatically controlling the dose. Hence, the unintercepted radiation increases detection errors of the dose, and the dose cannot be normally detected.
Normally, the front and side images of the breast portion must be photographed with different radiation qualities. However, at present, the operator must visually confirm the posture of the object and switch the radiation tube voltage at the console of the radiation generation device to photograph the front and side images of the breast portion.
In the radiography apparatus combining a radiation image intensifier and television camera, since radiographic observation is done on the television monitor prior to film photographing, the radiation range can also be visually confirmed. In this case, the operator need not stand at the side of the radiation generating means, and can also adjust the aperture stop by remote control. However, when the object region is to be extracted from the radiographic image of the object to automate aperture stop adjustment, the edges blur under the influences of scattered radiation and the like, thus making region extraction difficult. In addition, the object S is kept irradiated with radiation even during radiographic observation.
On the other hand, in the CR apparatus using the imaging plate, the signal level is detected by a coarse scan called a pre-scan using a very weak laser beam, so as to extract the object region, thereby optimizing the scan conditions for a main scan. However, since such processes are done after photographing the object S, they are not helpful in optimizing the photographing itself. Also, it is very hard to extract the object region under the influences of scattered radiation and the like as in the radiographic apparatus.
As described above, upon setting the photographing conditions and the like for the radiographic apparatus, the operator visually observes the object S, stops down the field of irradiation in correspondence with the size of the object S, adjusts the radiation quality in accordance with front and side shots, and manually switches the gain of a photo-timer.
However, since the radiographic apparatus depends on the operator to acquire information for recognizing the state and characteristics of the object S, it is especially difficult for the apparatus to accurately recognize the object region. The operator may often omit some setting operations in units of objects S, e.g., operation for stopping down the field of irradiation, to reduce his or her work loads. As a consequence, an image with inappropriate image quality may be obtained. For example, when radiation is irradiated even to an ineffective photographing region, the photo-timer produces recognition errors under the influences of unintercepted radiation, and a desired dose is not given. As a result, an effective radiographic image cannot often be obtained. Since the amount of radiation that reaches the human body differs depending on a front image in which the object S becomes thin or a side image in which the object S becomes thick, an effective radiographic image cannot be obtained unless the quality of radiation is switched, resulting in unnecessary radiation.
(e) In conventional art (5) above, in a photographing site with shorter photographing cycles, e.g., in group diagnosis, it is cumbersome to adjust the position of a lead aperture stop every time the object S changes. Especially, in breast photographing, the front and side images of one object S must often be alternately photographed, and adjustment must be done in each photographing. For this reason, photographing is often done with the aperture stop fully open. When a side image with a small width of the object S is photographed in such state, detection errors of the photo-timer of an automatic exposure means for automatically controlling the radiation dose increase under the influence of unintercepted radiation that is not absorbed by the human body, and an appropriate radiation dose often cannot be obtained. As a result, since sufficient object information cannot be obtained from the acquired image, photographing must be re-done.
Furthermore, although the radiation quality to be manually set must be optimized to improve image quality, since the optimal conditions vary depending on different portions, postures, and the like of the object, it is very cumbersome to set the quality in units of objects as in the lead aperture stop. For this reason, all objects are often photographed under identical conditions. Also, since the amount of scattered rays that reach the human body differs depending on the object postures in front and side shots, the gain of the photo-timer must be switched to obtain an effective radiographic image. In either case, improper photographing may result.
(f) In these conventional arts, it is consequently difficult to accurately photograph in correspondence with the object.
It is the first object of the present invention to provide a radiographic apparatus which can solve the problems in conventional art (a) above, and can shorten the operation time by simplifying cumbersome operations that must be done by the operator.
It is the second object of the present-invention to provide an X-ray photographing apparatus which determines the X-ray irradiation time by measuring the body thickness of an object before X-ray irradiation, and controls the moving grid on the basis of the determined X-ray irradiation time information to remove the influences of shadow images of lead foils formed in an X-ray image.
It is the third object of the present invention to provide a radiographic apparatus which can solve problems (c) in the above-mentioned conventional art, and can output an optimal image at high speed by performing image processes of a radiographic image.
It is the fourth object of the present invention to provide a radiographic apparatus which can solve problems (d) in the above-mentioned conventional art, and determines optimal photographing conditions by easily acquiring two-dimensional information of the object required for setting parameters for image processes of a radiographic image.
It is the fifth object of the present invention to provide a radiographic apparatus which can solve problems (e) in the above-mentioned conventional art, and can execute optimal radiography by acquiring object information immediately before photographing and reflecting it in setting of the photographing conditions.
It is the sixth object of the present invention to provide a radiographic apparatus which can execute appropriate radiography accurately corresponding to the situation of each object.